The present invention relates generally to wireless communication systems and more particularly to pilot sequence assignment in a cell free massive multiple-input multiple-output wireless system.
In a cell-free wireless communications system, access terminals are serviced by access points. In accordance with time division duplex (TDD) protocols, the access terminals synchronously and simultaneously transmit their pilot sequences to all access points. Each access point uses the pilot sequences to estimate channel coefficients between itself and each access terminal. High throughput in such a wireless communications system is achieved due to these estimated channel coefficients. The best estimates of channel coefficients can be obtained if the pilot sequences are orthogonal. Unfortunately, when the number of access terminals is large, the length of the pilot sequences must be increased to maintain orthogonality. However, the time required for long pilot sequences may be too long due to the limited length of the coherence interval. As such, short (and therefore non-orthogonal) pilot sequences are assigned to the access terminals. Current solutions for assigning non-orthogonal pilot sequences to access terminals result in a relatively low level of performance as compared to the ideal case when all pilot sequences are orthogonal.